Therapeutic Targeting of Microgliosis in Globoid Cell Leukodystrophy Globoid cell leukodystrophy (GLD), or Krabbe disease, is an often fatal genetic demyelinating disease resulting from loss of function mutations in the gene encoding galactosylceramidase (Galc). The consequence of mutated GALC enzymatic function is an accumulation of a lipid toxin called galactosylsphingosine, or psychosine. The cytotoxicity of psychosine is considered the basis of several key pathologies in GLD including severe demyelination, astrogliosis, axonopathy, and microgliosis. Presently, the only treatment for GLD patients is bone marrow transplantation (BMT), which unfortunately has limited efficacy. To better understand how astrocytes and microglia contribute to the pathogenesis of GLD, we have developed a new in vitro model in which to study glial interactions in response to psychosine. Using this model we have determined that psychosine potently induces astrocytes to produce matrix metalloproteinase-3 (MMP-3) which is required to activate microglia that can then kill oligodendrocytes. Expression of MMP-3 is dramatically elevated in brain tissues in the mouse model of GLD (?twitcher?) and its expression is not reduced in Twi mice by BMT. Thus, we believe we have identified a novel regulatory pathway to therapeutically target microgliosis which we hypothesize will mitigate pathology in GLD and enhance the therapeutic value of BMT to treat this disease. Our published and preliminary data support a pathway in which (a) psychosine induces the production of astrocytic MMP-3, that (b) increases miR-155-mediated repression of CD200 expression by astrocytes, and this (c) activates microglia that directly contribute to demyelination in this disease. We hypothesize that the direct contribution of psychosine-induced activation of microglia in GLD can be targeted to prevent the development of demyelination. In this proposal will determine the extent to which microglia contribute to this disease by investigating (Aim 1) the in vivo role of MMP-3 toward microglial activation and neuropathology in the twitcher mouse; (Aim 2) the in vivo role of CD200:CD200R1 signaling on microglial-mediated pathology in GLD; and, (Aim 3) the clinical utility of targeting microgliosis as a strategy to enhance bone marrow transplantation and reduce pathology in twitcher mice. Results from these studies will provide novel findings with potential therapeutic opportunities by testing the poorly understood contribution of microglia to GLD. Results from these collaborative studies are also expected to provide new information on a multi-step regulatory pathway that exists between astrocytes and microglia that can impact long-term treatment for diseases like GLD as well as a potentially wide range of other unrelated CNS conditions.